Functional Hierarchy of the Motor System Flashcards
What is the final comon path for the voluntary brain control of muscles?
Motoneurons in the spinal cord
What spinal level does reflex control of muscles occur?
Each segmental spinal level
Although reflex control occurs segmentally at each spinal level, what input does the brain have?
Brainstem nuclei exert control over spinal reflexes and integrate them into higher order reflexes to control posture and balance
What is an example of the brain regulating muscle reflexes?
Vestibulospinal and reticulospinal tracts regulate trunk and limb muscle reflexes
Where do the brainstem nuclei that regulate reflexes receive control inputs about voluntary movement from?
Higher centres:
cerebral cortex (motor, premotor and supplementary motor cortex)
Basal ganlia
Cerebellum
What 4 systems control movement?
Decending control pathways
Basal ganglia
Cerebellum
Local spinal cord/brain stem circuits
What function does the basal ganglia have for controlling movement?
Gating proper initiation of movement
What function does the cerebellum have for controlling movement?
Sensory motor coordination of ongoing movement
What function does the motor cortex have for controlling movement?
Planning, initiating and directing voluntary movement
What function does the brainstem have for controlling movement?
Basic movements and postural control
Where are the axons of the medial white matter from (descending)?
Brainstem
Where are the axons from the lateral white matter from (descending)?
Motor cortex
Are distal muscles controlled more by lateral or medial white matter?
Lateral white matter (from motor cortex)
Are proximal muscles controlled more by medial or lateral white matter?
Medial (brainstem)
What does the spinal cord receive descending input and direct cortical input via?
Descending - brainstem
Direct cortical - corticospinal (pyramidal) tract
What level does sensory input enter the spinal cord?
All levels
What kinds of sensory input enters at spinal cord level?
Proprioceptors
Touch
Pain
What kind of sensory input enters at the brainstem?
Vestibular system (informs about balance)
What kind of sensory input enters at cortical level?
Visual, olfactory, auditory, emotional and intellectual cues
What does damage to sensory inputs at spinal level lead to?
Paralysis as if the motoneurons themselves have been damaged
What system can be used for sensory information to allow movement if the proprioception is damaged?
Visual system
What is the simplest reflex and where is it found?
Stretch reflex which is found in all muscles
What is an example of the stretch reflex?
Patellar tendon or knee-jerk reflex
What does the stretch reflex follow?
A sharp tap to an inelastic tendon
Explain the process of the stretch reflex?
1) Sharp tap to inelastic tendon
2) Force transmitted to muscle fibres, which are more elastic so can stretch
3) Increases number of AP in afferent nerves projecting through dorsal horn into spinal cord
4) Divides and makes 3 types of connections
a) Activate a-motoneurons to stretched muscle causing rapid contraction of the agonist muscle
b) Connect indirectrly with and influence the antagonist muscle which relaxes due do spindle afferents activating inhibitory motoneurons which decrease activation of a-motoneurons to the antagnoist muscle
c) Spindle afferent information ascends in the dorsal column and makes connections in the somatosensory cortex to tell the brain about muscle length
What are the 3 connections spindle sensory afferents make in the stretch reflex?
Activate a-motoneurons to stretched muscle causing rapid contraction of the agonist muscle
Connect indirectrly with and influence the antagonist muscle which relaxes due do spindle afferents activating inhibitory motoneurons which decrease activation of a-motoneurons to the antagnoist muscle
Spindle afferent information ascends in the dorsal column and makes connections in the somatosensory cortex to tell the brain about muscle length
What is it called when fibres inhibit the antagonist muscle in the stretch reflex?
Reciprocal inhibition
What happens when a muscle relaxes?
Stretches
How many synapses occur for the spindle sensory afferents to activate moto-neurons to the agonist muscle in the stretch reflex?
Monosynaptic
Where do spindle afferents ascend to the somatosensory cortex in the stretch reflex?
Dorsal columns
What kind of feedback loop is the stretch reflex, explain this?
Negative feedback loop:
1) Muscle stretched stimulates muscle spindles
2) Causes reflex muscle contraction
3) Muscle shortens to previous length
What is the inverse stretch reflex also known as?
Golgi-tendon organ or clasp knife reflex
What is the inverse stretch reflex caused by?
Afferent nerves from the golgi tendon organs (GTO) which monitor muscle tension
What do the golgi tendon organs (GTO) monitor?
Muscle tension
Explain the process of the inverse stretch reflex?
1) Muscle contracts and shortens which pulls strongly on tendon and 1b sensory nerve from the GTO increase firing of APs
2) Causes 3 things:
a) Activation of inhibitory interneurons to the agonist muscle and a decrease in contraction strength
b) Activation of excitatory interneurons to the antagonist muscle
c) Information about muscle tenson ascends in the dorsal columns to the somatosensory cortex
3) Agonist muscle inhibited and relaxes rapidly, antagonist muscle is activated and contracts
What 3 things occur once the GTO increases the firing of APs in the inverse stretch reflex?
Activation of inhibitory interneurons to the agonist muscle and a decrease in contraction strength
Activation of excitatory interneurons to the antagonist muscle
Information about muscle tenson ascends in the dorsal columns to the somatosensory cortex
What does sensory information from the inverse stretch reflex use to reach the somatosensory cortex?
Dorsal columns
Is the inverse stretch reflex monosynaptic or polysynaptic?
Polysynaptic
What does the inverse stretch reflex prevent?
Muscles contracting so hard that the tendon insertion is torn away from the bone
What kinds of information does the flexor (withdrawal) reflex use?
Pain receptors (nociceptors) in skin, muscles and joints
Is the flexor reflex monosynaptic or polysynaptic
What does the flexor reflex do?
Withdraws part of the body away from a painful stimulus and in towards the body, so they flex the affected part
What kind of sensory nerves from the GTO increase firing of action potentials in the inverse stretch reflex?
1b sensory nerves
Explain the process of the flexor reflex?
1) Increased sensory action potentials from pain receptor cause
a) Increase in activity in flexor muscles of the affected part via a number of excitatory interneurons
b) At the same time, via a number of excitatory and inhibitory interneurons the antagonistic extensors are inhibited
2) Severe excitatory interneurons which cross the spinal cord excite the contralateral extensors
3) At the same time via several interneurons there is an inhibitions of the contralateral flexors
4) Sensory information ascends to the brain in the contralateral spinothalamic tract-
In the flexor reflex, how is it possible for increases sensory action potentials from pain to cause both increased in activity in flexor muscles and inhibition of extensors?
Small diameter Aδ nociceptive fibres triggering pain enter the cord, where they branch and activate interneurons in several spinal segments
What would happen during the flexor reflex if the contralateral limb did not extend?
You would fall over
What pathway does sensory information from the withdrawl reflex use to ascend to the brain?
Spinothalamic tract
Do nociceptive sensory fibres or muscle spindle afferents have a smaller diameter?
Nociceptive sensory fibres
Are nerves with a smaller diameter faster or slower than ones with a larger diameter?
Slower
What is a consequence of nociceptive fibres transmitting slower than muscle spindle afferents?
Flexor and crossed extensor reflex is far slower than stretch reflex
What can reflexes be overridden by?
Higher centres (consciously)
What is an example of a reflex being overridden consciously?
Load is excessive, GTO reflex is activated which drops load rapidly, but you can stop this and choose to over-ride it and voluntarily hold the load and damage your arm
What are examples of things that a single a-motoneuron synapses to?
The brain
Descending cortical excitatory and inhibitory signals
(so continual integration of EPSPs and IPSPs)
What does EPSP stand for?
Excitatory post-synaptic potential
What does IPSP stand for?
Inhibitory post synaptic potential
How can the stretch reflex be overridden?
Strong descending inhibition hyperpolarises a-motoneurons and the stretch reflex can not be evoked
